Gelled aqueous nitric acid composition and method of making same



United States 3,444,014 GELLED AQUEOUS NITRIC ACID COMPOSITION AND METHOD OF MAKING SAME Joseph D. Chrisp, Ashbourne Hills, Claymont, DeL, assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Filed Aug. 8, 1967, Ser. No. 659,034

Int. Cl. C06b N US. Cl. 149-45 14 Claims ABSTRACT OF THE. DISCLOSURE BACKGROUND OF THE INVENTION Liquid nitric acid finds use in the metallurgical, fertilizer, explosive, and general chemical process industries, where it serves principally in the preperation of inorganic nitrate salts or as a nitrating or oxidizing agent. However, use of the liquid acid as a chemical reactant or as an acidulating agent has always required special protection of both personnel and material, inasmuch as the inherent corrosiveness normally characteristic of a strong acid is accompanied by particularly strong oxidizing capacity. These protective considerations also apply in the case of chemical propellants and explosives in which the oxidizing power of nitric acid is harnessed by reacting it under controlled conditions with one or more fuels.

The potential of liquid nitric acid as sole or principal oxidizing component in such high-energy compositions apparently was first clearly recognized by Dr. Hermann Sprengel in the early 1870s. Since the time of Sprengels initial work on this type of charge, safer fuels and improvements in packaging and dispensing of the charges have been introduced, especially under the impetus of World Wars I and II, during which charges of the Sprengel type were of increased interest for military use.

However, in spite of these modifications, explosive containing liquid nitric acid have never received widespread acceptance, either for military purposes or in the civil explosives market. Principal reasons for this lack of acceptance are the corrosive nature of liquid nitric acid as well as the fumes (nitrogen oxides) that can evolve from the liquid acid under some conditions.

Another disadvantage of some of the explosives containing liquid nitric acid is their sensitivity to detonation by impact, or high shock sensitivity. Even when carefully prepared to avoid highly incompatible components and excessive heats of mixing, certain explosives of this type also have been known to fume off during formulation. Following Sprengels procedure of mixing the liquid acid and fuel at the time and site of use, including mixing in the blast hole in the case of use of the changes as blasting explosives, can minimize the more serious consequences of these problems but simultaneously may generate new ones. For example, in loading of the charges into blast holes containing appreciable amounts of water, sealing or packaging material that is watertight after several days or weeks exposure is generally necessary. Otherwise, ingress of water can lead not only to desensitization so that the charges cannot be detonated, but also to decomposition reactions associated with the high heat of dilution of nitric acid in Water. Even in so-called atent O "ice dry holes, losses of liquid components through subter ranean fissures or similar faults and reaction of the nitric acid with some minerals, e.g., carbonate-containing formations, decreases their effectiveness as explosives.

To permit storage of nitric acid explosives in the mixed state, several methods for immobilizing by thickening have been tried. For example, mineral substances of high absorptive capacity, such as kieselguhr and other diatomaceous earths, clays of various types, colloidal silicas, and, more recently, a number of high-molecular-weight organic compounds, particularly certain linear polymers of the vinyl type, have been added to the nitric acid compositions. However, none of these types of additives is inherently capable of giving products of high viscosity, cohesiveness or flexibility. Water resistance, a desirable characteristic especially in explosives to be used in wet, or potentially wet, locations, also is not markedly improved by their use. Furthermore, what thickening and stabilizing action is initially imparted by the linear polymer additives often is lost, in whole or in part, inasmuch as certain of these additives tend to undergo degradation in the presence of nitric acid.

SUMMARY OF THE INVENTION This invention provides a simple and effective means for obtaining modified aqueous nitric acid which is easier and safer to handle than that known heretofore and which has controlled free acidity and excellent stability.

Improved gelled nitric acid compositions are provided of the type comprising nitric acid having a srtength of about from to 99%, water, and a gelling system, the improvement comprising providing a gelling system comprising the in situ reaction products of:

(l) About from 0.5 to 10% by weight based on the weight of aqueous nitric acid of a polymer having recurring units of the general formula I CH2-C wherein R is selected from hydrogen, lower alkyl, and hydroxyalkyl having up to 4 carbon atoms and X is selected from and CO M, wherein M is selected from ammonium and alkali metal, and

(2) About 1 to 20% by Weight based on (1) of at least one metal ion having a positive valence of from 2 to 6 and selected from chromium vanadium, manganese, titanium, antimony, zirconium and scandium.

The invention also provides a process for the production of these compositions which comprises bringing into Contact:

(l) A nitric acid composition having a strength of about from 70 to 99%,

(2) A polymer having recurring units of the general formula i -CH2C wherein R is defined as above, and

(3) A salt of at least one of the metal ions specified above.

The invention further provides explosive compositions comprising the nitric acid compositions indicated above in addition to about from 5 to 30% of a fuel, up to 40% of a self-explosive sensitizer and up to 50% of an inorganic oxidizing salt.

The expression soluble in nitric acid and similar terms as used herein refer to components having appreciable solubility at ambient temperatures of (typically) 20 to 25 C. in the particular strength of aqueous nitric acid being employed. This solubility of the polymer should be at least about 10% by weight and preferably appreciably greater, e.g., at least about 50%. Furthermore the polymer should not precipitate from the aqueous nitric acid or agglomerate into flocculates or lumps. Crosslinking of the polymer by the polyvalent metal ions forms a structure which is a gel rather than a solid precipitate. This gelled structure swells in the aqueous nitric acid, holding the aqueous nitric acid, and is of substantially constant composition throughout. The gelled, crosslinked structure contains a fairly low ratio of polymer solids to liquid phase, e.g., less than about 1:10, this ratio being roughly equal to the weight ratio of the polymer to liquid phase, in contrast to precipitates or agglomerates in which there is a high ratio of polymer solids to liquid phase.

Reference to stability in nitric acid unless indicated otherwise indicates the absence of significant degradation of or attack on the polymer, or of the metallic ions, in the particular strength of aqueous nitric acid involved. Since prolonged stability often is of special significance in gelled compositions, stability refers particularly to absence of significant degradation of the gelled structure for a period of 4 to 12 hours.

Strength, as used with regard to nitric acid, expresses in percent the relationship between 100% (dry) nitric acid and the weight of nitric acid plus water in a particular acid. The aqueous nitric acid used in forming compositions of this invention generally will have a strength of about from 70 to 99%. Most effective use of the gel products, especially for explosive and propellant compositions is usually made when the strength of the acid is 75% or greater, particularly 75 to 97%, and this latter range of strengths is, accordingly, particularly preferred. In general, for a given amount of polymer employed, firmer gels are obtained in nitric acid of higher strengths.

The polymers which can be used in this invention comprise units derived from acrylic monomers of the formula F CHFC-X where R is hydrogen, lower alkyl or hydroxy alkyl having up to 4 carbon atoms; X is COOH, or COOM; and M is ammonium or an alkali metal. Examples of such monomers are amides such as acrylamide, rnethacrylamide, a-Z-hydroxymethyl acrylamide, or a-ethyl acrylamide; nitriles such as acrylonitrile, unethacrylonitrile, and a-butylacrylonitrile; acids such as acrylic acid per se, methacrylic acid, a-ethylacrylic acid and a-propyl acrylic acid; and salts such as sodium or ammonium acrylate and alkyl ammonium acrylates. Particularly preferred polymers from the viewpoint of solubility and commercial availability are homopolymers formed of acrylamide, acrylonitrile, or acrylic acid and copolymers of these monomers, particularly copolymers containing at least 5% of acrylonitrile or acrylamide. The polymers are preferably preformed, i.e., formed before their addition to the nitric acid, and have a molecular weight within the range of about from 1 to 6 million. Acrylamide polymers having a molecular weight of about from 5 to 6 million, such as commercially available from American Cyanamid as Cyanomer P-250, are particularly preferred. The higher molecular weight polymers, providing that they are soluble in the nitric acid of a particular strength, tend to provide firmer gels in shorter periods of time for a given weight of polymer added to the system than do polymers of lower molecular weight. When acrylonitrile polymers are used, they preferably have a molecular weight of about from 500,000 to 5 million. The polymers of acrylic acids and salts thereof employed usually will have a molecular weight of at least about 500,000.

The polyvalent metal ions are most conveniently added to aqueous nitric acid in the form of salts of which the particular anion is not critical. These metal ions can be provided, for example, by the nitrates, e.g.,

sulfates, chlorides, carbonates, acetates, chlorates, or perchlorates of the individual metals. Of these, the nitrates are usually preferred since they are readily soluble and since they contribute no foreign ions; however, vanadyl ions are most conveniently supplied by vanadyl sulfate, VOSO -2H O.

Preparation of the gelled nitric acid compositions of this invention can be accomplished by simply mixing of the nitric acid, optionally containing fuel or sensitizer components and dissolved salts such as ammonium nitrate, with the polymer followed by the addition of the source of crosslinking metal ions. Fuels or sensitizer components can alternatively be mixed with the polymer and this mixture then dispersed in aqueous nitric acid. When such additives are soluble in the system or easily dispersable and are of suflicient stability and inertness they normally are dissolved or dispersed in the aqueous nitric acid prior to the addition of polymer and source of the aforesaid polyvalent metal ions. When the additives are of marginal stability or have an inhibiting effect on ionization of the source of polyvalent metal ions or the crosslinking reaction, they usually are incorporated in the compositions after the composition has been gelled.

Solution of the polymer in nitric acid can be facilitated by first dissolving it in a small amount of water; such water is of course taken into consideration in determining the desired final acid strength. The mixing usually is conducted at ambient temperatures (20 to 25 C.), but temperatures down to the freezing point of the mixture can also be used without deleterious effects. Temperatures above about 40 C. should ordinarily be avoided in that at such temperatures hazardous, premature degradative oxidation of some fuels and sensitizers, including the acrylic polymers, can be initiated.

The viscosity of the gelled compositions can be varied according to the needs of a particular application. In general, the viscosity of the gel can be increased by increasing the strength of the acid, by increasing the percentage of polymer in the system, and by increasing the relative proportion of polyvalent metal ion used for a given quantity of polymer.

The compositions of this invention have greater homogeneity, resistance to disintegration or leaching by water, and stability, i.e., resistance to degradation and settling out of components, than compositions which are merely thickened. This greater homogeneity, stability and water-resistance is particularly advantageous when the gelled nitric acid compositions, which can contain added fuels and sensitizers, are to be used as explosives particularly in wet locations, since disintegration and leaching of a composition of water, if such occurs, can lead to failures to detonate or to propagate a detonation throughout the length of an explosive column. If the explosive structure degrades, i.e., by virtue of disintegration of the gel structure, subsequent segregation of components, particularly undissolved (solid) fuels and sensitizers, as discussed hereinafter, can occur under the force of gravity, and the components in a borehole, whether in a container or cartridge, shucked therefrom, or simply pumped into a borehole will become so heterogeneous that complete failure of detonation or propagation of detonation through the entire length of the column of explosive charge will occur.

The gelled nitric acid compositions of this invention can be employed for a variety of applications in which liquid aqueous nitric acid is ordinarily employed, e.g., as an oxidizing or nitrating agent in chemical synthesis, as an acidifying agent in mineralogical and other processes, in preparing nitrate salts, and in like operations. The gelled nitric acid compositions find particular merit in such applications when delayed action is desirable or required, inasmuch as the gels tend to release the acid slowly. However, the gelled nitric acid compositions of this invention have been found to have particular utility as explosives, and they are readily formulated to meet the specific requirements of such use. 7

Several of the gelled nitric acid compositions described above are inherently satisfactory as detonating explosives with further additives, i.e., they can be detonated With moderate-strength primers in diameters of 6 inches or less under moderate conditions of confinement, such as provided by a borehole or a container of moderate wall thickness. Preferably for explosive applications, the compositions of this invention also contain one or more fuels and/or nonexplosive sensitizers which are stable in the nitric acid of the strength used in preparing the gels. Examples of nonexplosive fuels are the monoand dinitroaromatic hydrocarbons, such as nitrobenzene, omononitrotoluene and dinitrotoluene; liquid and solid hydrocarbons and hydrocarbon fractions, particularly refined petroleum and mineral oils and the aromatic hydrocarbons, such as benzene, toluene, and the xylenes, carbohydrates, including various cellulose and starch products, e.g., cornstarch, potato starch, wood and paper pulps and sugar; siliceous fuels, including silicon itself and mixtures and alloys of silicon with heavy metals, e.g., ferrosilicon; and sulfur. Light metal fuels such as aluminum also are potentially useful in some of the gelled compositions, provided that they are, or can be made, sufficiently resistant to attack by the nitric acid. The gel copolymer per se acts as a fuel and except as otherwise indicated is included in calculating the amount of nonexplosive fuel and oxygen balance. Ordinarily, the gels for use as explosive compositions will be formulated to have an oxygen balance of about from 25 to +10%. Surfactants can be employed to insure complete dispersion of some fuels, e.g., the petroleum and mineral oils, in the explosive composition.

In addition to the nonexplosive fuels and/or sensitizers named above the explosive compositions of this invention can, if desired, contain an additive of the art-recognized self-explosive type, provided that such additive is stable in the strengths of nitric acid used in preparing the gels. TNT, for example, exhibits a high degree of stability in all strengths of aqueous nitric acid and hence is a particularly useful additive of the self-explosive type. Examples of other self-explosive components which can be used in the gelled nitric acid based compositions of this invention are RDX, nitrocellulose, smokeless powder, and other organic nitramines, nitrates and nitrocompounds. For reasons of economy and compatibility, TNT is preferred for use in the composition of this invention. The TNT or its mixtures (e.g., with ammonium or sodium nitrate) can be introduced into the compositions in the form of crystals, grains, pellets, flakes, or other particulate form which allows ready dispesrion thereof. In general, up to 85%; and preferably up to 40%, by weight of self-explosive additive based on weight of the composition is used.

As an additional component, the explosive compositions of this invention can contain up to about 85%, and preferably 5 to 50%, based on the total weight of composition, of an inorganic oxidizing salt, particularly an inorganic nitrate salt such as ammonium or sodium nitrate. The presence of the salt is beneficial in that it contributes to the total energy of the composition. At least part of the salt, when used, will be dissolved in the system; however, some of the salt can be undissolved providing that it is uniformly distributed throughout the gelled nitric acid matrix.

Stable, gelled nitric acid compositions found especially economical and efiicient as detonating explosives comprise a uniform blend of:

(a) About from 25 to 95% by weight of aqueous nitric acid having a strength of from about 70 to 99%, and preferably 75 to 97%;

(b) About from 5 to 30% of a nonexplosive fuel, preferably selected from siliceous fuels, light metals, liquid and solid hydrocarbons, carbohydrates, sulfur, mono and dinitroaromatic hydrocarbons, and mixtures of such fuels and/ or sensitizers; V

(c) Up to about 40% of a self-explosive sensitizer, particularly TNT;

((1) Up to about 50% of an inorganic oxidizing salt, typically, an inorganic nitrate; and

(e) A crosslinked gelling system of the type indicated above.

Agitation of the composition during formulation is usually continued until after the composition is gelled, particularly when added fuels and sensitizers are solids which must be uniformly distributed throughout the gel matrix. Where fuels or other additives of marginal stability are to be incorporated in the compositions, all ingredients except such additives can be mixed and gelled, then such additives added to the finished gel.

In general, the unit or bulk strength of an explosive composition based on gelled nitric acid increases with increasing strength of the nitric acid gelled. Accordingly gels of nitric acid of strength or higher are usually employed where high bulk strength is a requisite, e.g., in the bottom of a borehole. The bulk or unit strength of an explosive composition, its relative ease of initiation, and its minimum critical diameter can also be regulated to a large degree by the type and quantity of fuel and/or sensitizer employed. In general, a solid fuel such as ferrosilicon, sulfur or siliceous fuels is used to increase the bulk strength of a composition. Organic nitro compounds, typically mononitrotoluene, or dinitrotoluene or, in particular a self-explosive composition, especially TNT, are incorporated to provide compositions which are easily initiated, e.g., by a relatively small primer or by a blasting cap, in some cases, in small diameters. In many cases a combination of fuels will be employed Witihn the range of proportions indicated to give a composition having the desired physical and explosive properties.

Especially preferred gelling systems comprise polymers of acrylamide, acrylonitrile, acrylic acid and copolymers of these monomers. The polyvalent metal ion preferred is chromic (Cr+ In addition to being readily available at reasonable cost, these compositions are particularly effective in providing firm cohesive gels having viscosities Within the desired range of 100,000 to 5 million centipoises, high surface tension as evidenced by lack of tackiness, flexibility, water resistance and other desirable physical characteristics.

The compositions of this invetnion can be packaged in containers compatible with the gelled nitric acid, e.g., of polyethylene, polypropylene, or aluminum, and stored for several days without deterioration, gassing or separation of components.

Alternatively, the compositions can be prepared at the site of use and pumped or dumped directly into the borehole, which can be lined with a material such as polyethylene, which is both water-impervious and compatible with the gelled nitric acid. When such is the case, it is usually desirable that all components of the composition be liquid for ease of mixing and pumping the composition. Usually, the compositions to be pumped will be less viscous than those to be provided in containers and accordingly will comprise somewhat less of the polymer.

usually be provided with a protective coating such as polyethylene, polytetrafluoroethylene, polyoxymethylene, or polypropylene, which is resistant to attack by nitric acid. The primer charge used with the charge can be a pressed pellet, e.g., of TNT or RDX, or a gelled nitric acid based explosive composition made suitably sensitive to actuation, e.g., by Primacord, by the incorporation of such ingredients as grained TNT or by the use of acid of with mixing continued at a temperature of about C., except Where otherwise indicated. Then the source of polyvalent metal ions is added. Gelation is substantially instantaneous on addition of the source of polyvalent metal ions. None of the gel products shows visible indication of deterioration at ambient temperatures over periods of 8 hours or more.

Cap-sensitive explosive compositions are made with any of these gels by the admixture of a fuel, and optionally a self-explosive sensitizer and oxidizing salt. As indicated above, these components should preferably be admixed before the addition of the source of metal ion.

Similar results are obtained when the polymer used is a low molecular weight copolymer of acrylamide and acrylic acid commercially available from American Cyanamid as Cyanomer P-26. When scandium (Sc+ is used as the crosslinking agent, similar products to those of Example 16 are obtained.

TABLE 1 Polyvalent metal in ion, Acid Percent by percent by wt. strength, wt. of aq of acrylic Example percent Acrylic Polymer EN 03 polymer Gel product description 70 Polyacrylamide 1.44 Cr, 11.3% Weal: gel.

do 1.5 Cr ,11.4% Medium gel.

1.47 Cr+ 11.4% Medium gel.

1.43 Cr, 11.4% Firm gel.

2.25 Cr, 2.2% Weak gel.

2.25 Cr+ 11% Gel which was elastic and adhered to glass formed.

1.6 C1'+ ,6.5%. Weak gel. 2.4 Cr+ 7.5% Medium gel.

2.35 Cr, 10.8% Firm gel.

2.14 V, 17% Medi In gel, swelled by gas in 1-2 hr.

97 P0lyacrylonitri1e 1 Cr, 26% Wealr gel. 95 Polyacrylamide 2 3 Cr, 8% Medium finn gel. 98 do 2.2 Mn, 15% Medium weak gel.

1.5 Zr, 37% Firm gel. 1.5 Sb, 49% Medium gel. do 1.5 Ti, 21% Medium gel.

1 M01. wt. 1-2 million.

higher strength. Such primer charges of gelled nitric acid 40 compositions typically will be in a cartridge of a highstrength plastic such as polyethylene or polypropylene, which is also compatible with nitric acid.

In the following examples which illustrate this invention, parts, percentages and ratios are by weight unless otherwise indicated. In the following examples, the terms noted below mean the following:

Cheese-like.Viscosity of about from 10 million to 20 million cps. (centipoises) as measured with T F 1 spindle at 0.5 r.p.m. on the Brookfield Synchro-lectric viscometer.

Very firm.-Viscosity of about from 3 million to 10 million cps. with same conditions of measurement as for cheese-like gels.

Firm.Viscosity of about from 1 million to 3 million cps. measured with a TE spindle at 1.0 r.p.m. on a Brookfield Synchro-lectric viscometer, model RVT, with helipath attachment.

Medium firm.Viscosity of about from 400,000 to 1 million cps. using the same conditions of measurement on the viscometer.

Weak.Viscosities generally less than about 400,000 cps., generally 200,000 to 400,000 cps.

The following examples illustrate compositions of this invention containing various strengths of nitric acid and the use of various combinations of polymers and polyvalent metal ions.

Examples 1-16 Gelled aqueous nitric acid compositions are prepared from the material noted in Table 1. The polyacrylamide used in these examples is Cyanomer P-250, commercially available from American Cyanamid, having a molecular weight of 5-6 million. The nitric acid is charged to a mixing vessel then the polymer is added and dissolved 1 Changes in spindle and r.p.in, necessary to obtain ilt'it'lllzllu readings,

2 M01. wt., ca. 500,000.

I claim:

1. In gelled nitric acid compositions having a nitric acid strength of about from 70 to 99% and comprising nitric acid, and water, the improvement which comprises gelling said composition in situ with the reaction product of:

(1) about from 0.5 to 10% by weight based on the weight of aqueous nitric acid Of a polymer having recurring units of the general formula oin-2 1 wherein R is selected from hydrogen, lower alkyl, and hydroxyalkyl having up to 4 carbon atoms and X is selected from and CO M, wherein M is selected from ammonium and alkali metal, and

(2) about from 1 to 20% by Weight based on (1) of at least one metal ion having a positive valence of from 2 to 6 and selected from chromium, vanadium, manganese, titanium, antimony, zirconium and scandium.

2. A composition of claim 1 wherein said composition comprises about from 1.0 to 5% by Weight, based on the nitric acid, of the polymer and about from 2 to 12% by weight, based on the polymer, of the metal ion.

3. An explosive composition of claim 2 wherein said nitric acid composition further comprises about from 5 to 30% of nonexplosive fuel.

4. A composition of claim 3 wherein the nitric acid composition further comprises up to 40% of a selfexplosive sensitizer and up to 50% of an inorganic oxidizing salt.

5. A composition of claim 4 wherein the self-explosive sensitizer is trinitrotoluene and the inorganic oxidizing salt is an inorganic nitrate.

6. A composition of claim 3 wherein said polymer comprises polyacrylamide.

7. A composition of claim 3 wherein said polymer comprises polyacry1onitrile 8. A composition of claim 3 wherein said polymer comprises a copolymer of acrylamide and acrylonitrile, wherein each component comprises at least of said polymer.

9. A composition of claim 3 wherein said polyvalent metal ion is chromium.

10. A composition of claim 3 wherein said polyvalent metal ion is vanadium.

11. A composition of claim 3 wherein said polyvalent metal ion is manganese.

12. A process for the production of gelled aqueous nitric acid compositions which comprises bringing into contact:

(1) a nitric acid composition having a strength of about from 70 to 99% (2) a polymer having recurring units of the general formula wherein R is selected from hydrogen, lower alkyl,

and hydrovylalkyl having up to 4 carbon atoms and X is selected from and CO M, wherein M is selected from ammonium and alkali metal, and

(3) the salt of at least one metal ion having a positive valence of from 2 to 6 and selected from chromium, vanadium, manganese, titanium, antimony, zirconium and scandium,

13. A process of claim 12 wherein the nitric acid composition and the polymer are admixed and the metal ion brought into contact with the resulting mixture.

14. A process of claim 13 wherein about from 5 to 30% of a nonexplosive fuel, up to of a self-explosive sensitizer and up to of an inorganic oxidizing salt are admixed with the compositon before the addition of the metal ion.

References Cited UNITED STATES PATENTS 3,282,754 11/1966 Gehrig 14974 3,296,044 1/1967 Gehrig 14974 X 3,306,789 2/1967 Logan et a1. 149-56 X 3,336,981 8/1967 Barron et a1 14974 X 3,369,943 2/1968 Longwell et al 149-74 X 3,376,176 4/1968 Gehrig 14974 X 3,361,601 1/1968 Chrisp 149-74 CARL D. QUARFORTH, Primary Examiner.

S. J. LECHERT, Assistant Examiner.

US. Cl. X.R. 

